One of pollutants emitted to the air through fuel combustion is nitrogen oxide (NO, NO2, NO3, N2O, N2O3, N2O4, or N2O5). Nitrogen oxide causes, for example, acid rain, ozone depletion, and photochemical smog and seriously affects the environment and the human body, and therefore the treatment for nitrogen oxide has been an important issue.
A known technique of removing the nitrogen oxide is a selective catalytic reduction reaction (NH3-SCR) that uses ammonia (NH3) as a reducing agent. As described in Patent Document 1, a catalyst in which vanadium oxide is supported on titanium oxide serving as a carrier is widely used as a catalyst for the selective catalytic reduction reaction. Titanium oxide is the best carrier because titanium oxide has a low activity against sulfur oxide and has high stability.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-275852